1. Field of the Invention
The present invention relates to powder coating compositions, specifically to powder coating compositions containing additives that improve the transfer efficiency of the powder coating compositions.
2. Description of the Related Art
It is known that during the application of a powder-coating composition on to substrate, not all of the coating composition ends up on the substrate. The amount of powder-coating material being applied on to a substrate that actually ends up on the substrate as the powder coating relates to the transfer efficiency of the process. The transfer efficiency can be calculated by dividing the change in weight of the substrate by the change in weight of powder coating source dispensing the powder coating composition wherein the change in weight is the weight difference before and after the application of the powder coating. The transfer efficiency is conveniently expressed as a percent.
Although the powder coatings market is growing rapidly, one of the limiting factors for continued growth is the transfer efficiency. Typical powder coating application lines have first pass powder transfer efficiency from spray point to substrate of about 50-80%. The remaining powder has to be reclaimed and re-sprayed. Although this respray ability enables the powder coatings industry to cite lower coating costs and higher utilization of raw materials (of up to 95%) than liquid, solvent-based coatings it does decrease the efficiency of the process.
Furthermore, in some applications recycling powder is not possible owing to issues relating to appearance and quality of the finished part. One such area is in automotive clear coat applications. In these applications, the recycled material may contain lint or other foreign matter that is likely to cause defects in the finished coated automotive part.
Also, when using metallic or other dry blended systems (expensive color, etc.), the components may not transfer at equal rates (for example, metallic pigments or colors transfer at a different rate than the rest of the coating). Any reclaimed material looks different than the originally transferred coating. Thus spraying reclaimed material here amounts essentially to spraying to waste. This adds further cost to an already expensive dry blending process that is used to mix some of the components together to make the coating. Clearly there is a need for improving the transfer efficiency of powder coatings. The present invention fulfils such a need.
A transfer efficiency additive can positively affect the economics of the powder coating process. If recycling and reclaiming of powder-coating compositions are not feasible or desired, as mentioned above for automotive clear coats or when using metallic colors, then an efficient first-pass transfer becomes even more important. Two other factors that could be affected by a transfer efficiency additive are: 1) an improvement in the ability of the powder spray to penetrate into corners and close spaces, and 2) improving the ability of very small size powder coating particles to efficiently transfer and build on a metal substrate. Some industry sources estimate that an increase of about 3 to 5% in transfer efficiency would justify the cost involved in adding a transfer efficiency additive to achieve this increased transfer efficiency.
U.S. Pat. No. 5,073,579, Macholdt et al., Dec. 17, 1991, discloses a process for enhancing the electrostatic chargeability of powder coatings or powders intended for surface-coating solid objects, by homogeneously incorporating at least one triarylmethane, azine, thiazine and/or oxazine compound in an amount from about 0.01 to about 10 percent by weight into powder coatings or powders, the compounds mentioned being dissolved or dispersed in the powder coating system or powders.
U.S. Pat. No. 5,021,473, Macholdt et al., Jun. 4, 1991, discloses a triboelectrically sprayable powder coating composition consisting essentially of
(1) an epoxy resin; or PA1 (2) a hydroxyl- or carboxyl-containing polyester resin; or PA1 (3) an acrylic resin; or PA1 (4) a combination of hydroxyl-or carboxyl-containing polyester resin and epoxy resin, or combinations thereof, and, homogeneously distributed, dissolved, or dispersed in said powder-coating composition, about 0.01 to about 10 percent by weight of at least one salt-like cationic compound of the formula (1) ##STR1##
in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4, independently of one another, denote hydrogen atoms, straight-chain or branched alkyl groups having 1 to 30 carbon atoms, mononuclear or polynuclear cycloaliphatic radicals having 5 to 12 carbon atoms, oxethyl groups of the formula --(CH.sub.2 --CH.sub.2 --O).sub.n --R in which n denotes a number from 1 to 10 and R denotes a hydrogen atom, a (C.sub.1 -C.sub.4)-alkyl or acyl group, furthermore denote a mononuclear or polynuclear aromatic radicals or araliphatic radicals, where the aliphatic, aromatic and araliphatic radicals may be substituted by hydroxyl, (C.sub.1 -C.sub.4)alkoxy, secondary or tertiary amino groups, acid amide groups and/or acid amide groups, where the alkyl, cycloalkyl, aralkyl and aryl radicals for R.sub.1 to R.sub.4 may be substituted by fluorine, chorine or bromine atoms, X denotes a phosphorus, arsenic or antimony atoms, where, in the case where X denotes an arsenic or antimony atom, at least one of the radicals R.sub.1 -R.sub.4 does not denote a hydrogen atom, and A.sup.- denotes the equivalent of an inorganic or organic anion. This patent also discloses a method for enhancing the electrostatic chargeability of a triboelectrically sprayable powder-coating composition comprising the above composition in an amount from about 0.01 to about 10 percent by weight into powder coatings.
European Patent Publication 0 371 528 Al, Stamicarbon, Jun. 6, 1990, discloses a triboelectrically processable powder coating based on a polyester-containing binding agent and a nitrogen-containing additive, characterized in that the nitrogen-containing additive is a sterically hindered tertiary amine or aminoalcohol.
U.S. Pat. No. 4,908,225, Niimura et al., Mar. 13, 1990, discloses a method of electrostatic coating, which comprises electrostatically coating a surface with a resin powder composition, wherein said composition comprises a thermosetting or thermoplastic resin the improvement comprising incorporating into said resin, an electric charge-increasing agent selected from the group consisting of a phthalocyanine compound, an anthraquinone compound, a metal compound of an organic carboxylic acid and an alkyl onium salt.